In IEEE 802.11 wireless LANs, automatic power save delivery (APSD) is a mechanism that aims to reduce power consumption by the devices connected in the network. It was originally introduced to IEEE 802.11e in 2005.TechniqueAPSD is generally deployed in infrastructure BSS (basic service set). In infrastructure BSS, there is are access points (APs) which are basically wireless routers forming the base stations for access. All the wireless devices i.e. clients communicate with each other through the APs. When a client has frames to send it sends the frames to the corresponding AP along with the destination address. The AP then ... Read More

In computer networks, beacon frames management frames used in IEEE 802.11 based wireless local area networks (WLANs). A beacon frame contains network information needed by a station before it can transmit a frame. They are used for announcing the presence of devices in a WLAN as well as synchronisation of the devices and services.Beacon frames are used as a part of BSS (basic service set). In infrastructure BSS, there is are access points (APs) that are wireless routers forming the base stations for access. All the devices communicate with each other through APs. The APs transmit beacon frames periodically so ... Read More

Virtual channel sense or virtual carrier sense is a mechanism to predict future traffic in wireless networks that uses carrier sense multiple access with collision avoidance (CSMA/CA). It is implemented in wireless network protocols, IEEE 802.11 and IEEE 802.16, which operates in the medium access control (MAC) layer.In virtual channel sensing, a timer mechanism is used that is based upon information of durations of previous frame transmission in order to predict future traffic in the channel. It uses network allocation vector (NAV), which can be considered as a counter that counts down to zero.TechniqueVirtual channel sensing mechanism initialises a NAV ... Read More

The network allocation vector (NAV) is a virtual carrier sensing mechanism that forms an important part of the carrier sense multiple access with collision avoidance (CSMA/CA). It is implemented in wireless network protocols, IEEE 802.11 and IEEE 802.16, which operates in the medium access control (MAC) layer.TechniqueThe network allocation vector can be considered as a counter that counts down to zero. The maximum NAV duration is the transmission time required by frame, which is the the time for which the channel will be busy. At the start of transmission of a frame, the NAV value is set to its maximum. ... Read More

Point coordination function (PCF) is an optional technique used to prevent collisions in IEEE 802.11-based WLAN standard including Wi-Fi. It is a medium access control (MAC) sublayer technique used in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used.PCF is used additionally along with the mandatory distributed coordination function (DCF). It is used in centralised control system, and is present in the access point (AP) of the wireless network. An AP is generally a wireless router that coordinates network communication.Features of PCFIt is an optional function that resides on the top of the mandatory DCF. Both PCF and ... Read More

Distributed coordination function (DCF) is a mandatory technique used to prevent collisions in IEEE 802.11-based WLAN standard (Wi-Fi). It is a medium access control (MAC) sublayer technique used in areas where carrier-sense multiple access with collision avoidance (CSMA/CA) is used.TechniqueThe basic technique of DCF is as follows −When a station has a frame to transmit, it waits for a random backoff time. The random backoff time is defined by a contention window having a random number of time slots. The backoff time is given by the following equation −Timebackoff = random()×TimeslotsHere, the function generates a random number and is the ... Read More

IEEE 802.11 standard, popularly known as WiFi, lays down the architecture and specifications of wireless LANs (WLANs). WiFi or WLAN uses high frequency radio waves instead of cables for connecting the devices in LAN. Users connected by WLANs can move around within the area of network coverage.The 802.11 MAC sublayer provides an abstraction of the physical layer to the logical link control sublayer and upper layers of the OSI network. It is responsible for encapsulating frames and describing frame formats.MAC Sublayer frame of IEEE 802.11The main fields of a frame of wireless LANs as laid down by IEEE 802.11 are ... Read More

In wireless communications, when multiple antennas are used both at the transmitting end as well as the receiving end, the configuration is said to be multiple input, multiple output or MIMO. MIMO helps in sending and receiving multiple data signals simultaneously over the same radio channel. MIMO is a smart antenna technology, the other popular technology being multiple input, single output (MISO) and single input, multiple output (SIMO).The following diagram shows MIMO configuration −Advantage of MIMO over Traditional Wireless SystemsIn traditional wireless communication systems, a single antenna is used at the transmitter site, and another single antenna is used at ... Read More

Here we will see the md5sum command in Linux system. The MD5 is one of the Message Digest algorithm, that generates hash values to create checksum for the messages. It helps to identify if the integrity is maintained or not. Here we will see some example of md5sum command in Linux system.If we use this command on a string, it will generate a hash value, if we create some minor change in the text, it will generate massive change in the checksum.Examplesh-4.4$ echo 'Hello World' Hello World sh-4.4$ echo 'Hello World' | md5sum e59ff97941044f85df5297e1c302d260 - sh-4.4$If we change the content ... Read More

An operating system (OS) is basically a collection of software that manages computer hardware resources and provides common services for computer programs. The operating system is a crucial component of the system software in a computer system.These are some few common services provided by an operating system −Program executionI/O operationsFile System manipulationCommunicationError DetectionResource AllocationProtectionIn the matter of multi-user or multi-tasking environments, resources such as main memory, CPU cycles and files storage are to be allocated to each user or job. Some major activities of an OS with respect to resource management are −The Operating System manages all kinds of resources ... Read More